The present invention relates to ink jet printing and deals more specifically with apparatus for vertically randomizing the flight path of an ink drop ejected from a printing means.
Graphics, particularly large-scale color graphics, such as outdoor advertising billboards and signs or displays used in large open areas such as shopping malls and airports, produced by an ink jet system are often not of high quality and have a corduroy texture or washboard appearance.
One ink jet printing system for producing large-scale graphics moves a receiving surface relative to an ink jet printing station in a continuous, line scanning fashion to print a line. The printing station generally has a number of ink jet printing heads which may be arranged to print the same color for monochromatic graphics or which may be arranged to print a number of colors in a polychromatic halftone manner such as generally described in prior U.S. Pat. No. 4,367,482 to produce polychromatic graphics. In actuality, each printed line is really a horizontal band which is made up of a number of pixel areas arranged end-to-end and located sequentially along the scan line. Potential dot positions form an array of rows and columns identical for all pixel areas and each row of the array is associated with one printing head of a group of heads comprising the printing station. As each printer head moves along a scan line it moves past a succesion of points on the line in relation to each of which the printer head may (or may not) eject a relatively large volume drop of ink to apply dots of substantially fixed size onto the surface at the dot position. At the completion of the printing of the line, the printing station moves downwardly a distance equal to the height of the printed line and the next group of lines associated with the rows forming a pixel area is printed immediately adjacent to the previous group. A large number of such side-by-side printed lines form the desired sign or display.
An ink drop printed at a dot position on the surface is not a uniform thickness due to the thixotropic properties and surface tension generally characterizing pigmented inks and exhibits density variations across its surface with the density being higher at the dot center than at its periphery. Consequently, a printed line may exhibit a lower density along its edges than at its center and the region or gap between adjacent printed lines may be lighter than the centers of the lines. The repetitive lighter gaps can produce a corduroy texture appearance in the completed graphic.
It is therfore a general aim of the present invention to provide an ink jet printing apparatus for vertically randomizing the flight path of an ink drop to print dots in a vertically randomly deviated manner with respect to a line scanned by an ink jet head to substantially eliminate the corduroy texture appearance that is produced by the repetitive lighter gaps between adjacent printed lines.
Other objects and advantages of the invention will become readily apparent from the following description and claims taken in conjunction with the accompanying drawings.